Around 29,000 lightning strikes lit up the sky over southern England overnight, as thunderstorms caused flash flooding, travel chaos and at least two house fires in London. The storms rolled in from the south-west on Monday evening and moved east, waking thousands with loud thunder and brilliant flashes. It came as the UK braced for an extreme heatwave, with temperatures forecast to reach 40C and a rare red weather warning issued for parts of England and Wales.
Thunderstorms and heatwaves are closely linked. The Met Office explains that the two vital ingredients for a thunderstorm are instability in the atmosphere and moisture. During a heatwave, the ground heats the air above it, creating a layer of warm, humid air near the surface. If a trigger — such as an atmospheric disturbance higher up — allows that warm air to rise rapidly, it can form towering cumulonimbus clouds. Inside these clouds, particles of ice move and bump into each other, building up an electric charge that can suddenly release as a lightning strike, heating the air to around 10,000C and producing thunder. The energy released can also cause intense downpours.
“Why heatwaves often trigger thunderstorms in the UK, with science, history and safety advice.”
This pattern is well known in UK weather history. During the famous 1976 heatwave, the Met Office recorded that "thunderstorms occurred in many districts" in early July, causing local flooding. On 12 July 1976, a man was killed when his lorry was struck by lightning in Clapham, Bedfordshire, and 64.8mm of rain fell at Sudborough, Northamptonshire. The current heatwave is expected to surpass the June temperature record set in Hampshire in 1976, with the UK's all-time high of 40.3C (measured in July 2022) possibly within reach. The Met Office's Grahame Madge noted that the combination of high temperatures and humidity makes the heat even more impactful: normally the body cools through evaporation, but when the air is already saturated with water vapour, sweat cannot evaporate.
For UK readers, understanding this cycle matters directly to daily life. The recent storms caused flooding on roads and at Heathrow Airport, disrupting the Elizabeth line and other Tube services. London Fire Brigade said it took about 400 calls overnight, including two house fires believed to be caused by lightning. The red extreme heat warning, only the second ever issued by the Met Office, covers an area from London to Swansea and Somerset to Birmingham, urging people to stay hydrated, avoid the sun during the hottest part of the day, and look out for elderly relatives and those with health conditions. Overnight temperatures in urban areas may not drop below 20C — a "tropical night" that makes recovery harder. The UK Health Security Agency warns that even healthy individuals are at risk during a red alert.
Q: Why do thunderstorms often happen after a heatwave in the UK? A: Heatwaves warm the ground and lower atmosphere, creating a large store of energy and moisture. When a weather disturbance passes overhead, it triggers the rapid upward movement of that warm, humid air, which condenses into thunderclouds. This is why intense thunderstorms frequently develop in the late afternoon or evening after a hot, humid day.
Q: How many lightning strikes hit the UK during the recent storm? A: The Met Office recorded 29,074 lightning strikes across England in the 24 hours to 9am on Tuesday, with most (18,540) over Somerset. This included cloud-to-cloud and cloud-to-ground strikes. While the total number is not unusual for a storm after a hot day, the frequency and intensity were described as spectacular.
Q: What should I do to stay safe during a thunderstorm and heatwave? A: During a thunderstorm, avoid open areas, tall objects, and water. If you are indoors, unplug appliances and avoid using landline phones. For heatwaves, the UKHSA advises staying hydrated, keeping your home cool, and avoiding the sun between 11am and 3pm. Follow the latest weather warnings from the Met Office and check on vulnerable neighbours.
What happens next? The Met Office forecasts that temperatures will peak on Wednesday and Thursday, possibly reaching 39-40C in London and the South East. By Friday, a shift in weather patterns is expected to bring cooler air from the west, with highs dropping towards the high 20s Celsius over the weekend. However, more thunderstorms could develop as the heat eases. The Foreign Office has also issued extreme weather warnings for Britons travelling to France, Spain and Portugal, where a "heat-dome" is driving temperatures above 40C.